I think that was because I had to build Kodi and mpv with different ffmpeg versions at the time, because mpv was updated to ffmpeg 7 but Kodi wasn't ready yet.
Similar situation like today with ffmpeg 8. There exists a patch to add ffmpeg 8 support in kodi but to me it feels like its a bit to early for this step. 

 couldn't find a way to use two different system versions of ffmpeg as long as they only differed in the version number.  

I only remember that I had issues with using it but not exactly what it was.